Forced circulation lubricating systems are commonly used for lubricating of various rotational and moving parts of combustion engines. Typically lubricating medium is fed by means of a delivery ductwork system from a container to lubrication targets, after which the lubricating medium is accumulated and led back to the container. In modern diesel engines the pressure for circulating the lubricating medium is provided by oil pumping means. Typically an oil pump sucks oil from the oil sump of the engine and pumps the oil through a cooler and filter to the ductwork or channels leading to the various lubrication targets. From the lubricated targets the oil drains back to the bottom of the crankcase to the oil sump. The system is designed so that the pressure of the oil is suitable for lubricating the lubrication targets. However, with this known kind of system, there are the disadvantages that the pressure of the oil is dependent on the rotating speed of the engine and the oil pressure is approximately constant at various parts of the lubricating system. Thus the system does not take into account specific requirements for different lubrication targets or operational circumstances.
Specifically the feeding of lubricating oil for piston units includes drawbacks in the case of large two-stroke engines. In EP-A-0903473 there is shown a lubricating system for a large two-stroke engine, in which the lubricating medium is delivered to the piston by means of separate linked levers provided with flow channels. A separate oil feeding device is provided for each cross head, which device operates by the effect of reciprocating movement of the cross head. In this known system the feed of lubricating medium is also set to a certain level, which is not adjustable during the operation of the engine and thus does not necessarily correspond to the optimum lubrication for all operating circumstances.
Another known lubricating system is shown in GB-A-2058952. This known lubricating system utilises fully electronically controlled valves. However this known solution is unnecessarily complicated.
The main bearings of engine crankshafts are usually hydrodynamic bearings. The efficiency of lubricating such bearings is based on creating a film of oil between the bearing surfaces. The settling of the shaft is dependent on, among other matters, the peripheral velocity of the shaft pin. Thus the lubrication is not ideal, for example, during engine start-up.
It is an aim of the present invention to provide a lubricating arrangement for a piston engine which minimises the shortcomings of the prior art. Specifically an aim of the invention is to provide a lubricating arrangement by means of which the feeding of lubricating medium for lubricating engine parts, e.g. pistons, takes place reliably and efficiently. It is also an aim of the invention to provide a method of lubricating a piston engine, in which shortcomings of the prior art are minimised.